This invention relates to display filter arrangements and to display assemblies including filter arrangements.
Optically-emitting displays, especially in aircraft applications, often require optical filtering, such as for contrast enhancement or for the selection of particular color or wavelength bands. In aircraft applications, displays have to be clearly visible throughout a wide range of different ambient lighting conditions ranging from darkness to full sunlight, which might be 1500001 lux. In conditions of high illumination, there is a problem that ambient light incident on the display will be reflected from the cover glass, filters or other surfaces and will swamp out the light emitted from the display itself, rendering it illegible. The amount of light reflected from the surfaces of the display can be greatly reduced by the use of anti-reflection coatings. These, however, cannot be used on every surface, such as the internal surface of a CRT which is coated with a phosphor layer. To overcome the problem of reflection from these layers, it is usual to employ some form of contrast enhancement technique. This may take the form of a filter that absorbs all visible radiation except that emitted by the display. Alternatively, a circularly polarizing filter can be used to remove specular reflections from any surface behind the filter. Although these techniques help to improve the readability of the display, they do not provide a complete solution over the range of ambient lighting conditions likely to be encountered.
In aircraft or other vehicle applications, displays may need to be compatible with night-vision goggles. These goggles improve the ability of the pilot or other wearer to see poorly illuminated objects outside the aircraft, by amplifying what low levels of radiation are received by the goggles. Such goggles can have a gain of about 2000 and typically are responsive to radiation in the wavelength range of 620 nm to 930 nm. In order to prevent light emitted by the aircraft displays swamping the goggles, it is necessary for the light emitted by the displays to be confined outside this range of wavelengths, such as by a suitable choice of light-emitting diode (LED) elements and phosphors etc together with filters in front of the displays that remove light in the range to which the goggles are sensitive. For example, with a green LED display the emission peak is 570 nm which is close to the shorter wavelength end of the goggle characteristics. Because the emission of LED's is not narrowly defined, there will be overlap, at the longer wavelengths, with the goggle's characteristics. One solution would be to use absorptive filters to remove unwanted radiation with wavelengths greater than 620 nm. The problem, however, with most filters is that they do not have a very sharp cut-off. If, therefore, the radiation to which the goggles are sensitive is to be reduced sufficiently to prevent swamping the goggles, the effect of the filter will also be to reduce appreciably the amount of light transmitted through the filter at the LED's emission peak. This, consequently, reduces the visibility of the display in normal viewing conditions, especially when there is high ambient lighting. There are also other applications where it is necessary to provide a highly selective filter, such as in triple band pass filters used with color CRT displays.